Begin Architecture changes for TFA module

[doylejd]

• The start of major architectural changes for Tfa.
  • Since we are using plugins, we don't need the Tfa class anymore. The Dependency Injection of PluginManagers can handle loading the proper plugins based on the global (or eventually user) settings.
  • Simplify the process and have TfaLoginForm/EntryForm will handle the loading of plugins that it uses (Validation and Login) rather than calling a service which calls the deprecated Tfa.php class.
• Started deprecating the Tfa.php and TfaManager.php files.
• Began work on Trusted Browser plugin to integrate login plugins.